I have added libmagic too. I forgot to mention it 
It is described in the Readme on the Github repo. I already use it in my plugin File MIME type column.
MediaInfoLib
I'm looking into it. I have checked the existing plugin MediaInfo-based extended columns, it is gigantic
I need to see if there will be any benefit of having the functionality inside a COM dll.
There is another idea I got. Apart from libmagic, there is also TrID, which provides more comprehensive information about file type. It relies on a big database. COM DLL can allow the database to be loaded once, and then every request for a file type in DOpus will be very fast.